1. Field of the Invention
Exemplary aspects of the present invention relate to an image forming apparatus and a developing device. One particular aspect of the invention relates to an image forming apparatus and a developing device for developing a latent image with a developer.
2. Description of the Related Art
A related art image forming apparatus, such as a copying machine, a facsimile machine, a printer, or a multifunction printer having copying, printing, scanning, and facsimile functions, forms a toner image on a recording medium (e.g., a sheet) according to image data by an electrophotographic method. For example, a charger charges a surface of a photoconductor serving as a latent image carrier. An optical writer emits a light beam onto the charged surface of the photoconductor to form an electrostatic latent image on the photoconductor according to image data. A developing device develops the electrostatic latent image with a developer containing toner particles and magnetic carriers to form a toner image on the photoconductor. The toner image is transferred from the photoconductor onto an intermediate transfer member and is further transferred from the intermediate transfer member onto a sheet. A fixing device applies heat and pressure to the sheet bearing the toner image to fix the toner image on the sheet. Thus, the toner image is formed on the sheet.
One example of the developing device includes a developer carrier, a supply-convey screw opposing the developer carrier, and a convey screw. The supply-convey screw supplies a developer to a whole area in a longitudinal direction of the developer carrier, while the supply-convey screw conveys the developer along its axial direction. While supplying the developer to the developer carrier, the supply-convey screw collects a developer used for developing an electrostatic latent image from the developer carrier. The supply-convey screw conveys the collected developer to its downstream end in a developer conveyance direction of the supply-convey screw. At the downstream end, the developer is delivered from the supply-convey screw to an upstream end of the convey screw in a developer conveyance direction of the convey screw. While the convey screw conveys the developer to its downstream end in the developer conveyance direction of the convey screw, the developer is mixed with a replenishing developer and is delivered to an upstream end of the supply-convey screw in the developer conveyance direction of the supply-convey screw. In the above-described circulation of the developer, while the supply-convey screw collects a developer having a decreased toner density due to development from the developer carrier, the supply-convey screw supplies a developer replenished with toner particles and thereby having a recovered toner density to the developer carrier.
However, the developer conveyed on the supply-convey screw has various toner densities. For example, the developer carried on the upstream end of the supply-convey screw in the developer conveyance direction of the supply-convey screw contains more replenishing toner particles than the developer carried on the downstream end of the supply-convey screw in the developer conveyance direction of the supply-convey screw. On the contrary, the developer carried on the downstream end of the supply-convey screw in the developer conveyance direction of the supply-convey screw contains more developer collected from the developer carrier after being used for development than the developer carried on the upstream end of the supply-convey screw in the developer conveyance direction of the supply-convey screw. Namely, the developer carried on the downstream end of the supply-convey screw in the developer conveyance direction of the supply-convey screw has a lower toner density than the developer carried on the upstream end of the supply-convey screw in the developer conveyance direction of the supply-convey screw.
Another example of a developing device includes a screw for collecting a developer from a developer carrier and another screw for supplying a developer to the developer carrier. For example, the developing device includes a supply-convey screw, a receive-convey screw, and a slant-convey screw. The supply-convey screw supplies a developer to the developer carrier. The receive-convey screw is disposed immediately under the supply-convey screw. The slant-convey screw extends obliquely relative to a horizontal direction in which the supply-convey screw and the receive-convey screw extend. The receive-convey screw receives a developer used for developing an electrostatic latent image from the developer carrier, adds toner particles to the developer so that the developer has a recovered toner density, and sends the developer to the slant-convey screw. The slant-convey screw receives the developer sent from the receive-convey screw and conveys the developer up to the supply-convey screw disposed directly above the receive-convey screw.
A downstream end of the receive-convey screw in a developer conveyance direction of the receive-convey screw extending in the horizontal direction is located at a height level substantially common to an upstream end of the slant-convey screw in a developer conveyance direction of the slant-convey screw extending obliquely relative to the horizontal direction. Namely, the slant-convey screw extends upward from its upstream end to its downstream end in its developer conveyance direction. The developer is delivered against gravity from the receive-convey screw to the slant-convey screw via an opening provided in a wall disposed between the receive-convey screw and the slant-convey screw. Specifically, the downstream end of the receive-convey screw in the developer conveyance direction of the receive-convey screw opposes the upstream end of the slant-convey screw in the developer conveyance direction of the slant-convey screw via the opening. Since the slant-convey screw is obliquely disposed relative to the receive-convey screw, a part near the upstream end of the slant-convey screw in the developer conveyance direction of the slant-convey screw, which faces the receive-convey screw via the opening, is located at a height level higher than the receive-convey screw. Therefore, a developer is delivered against gravity from the receive-convey screw to the slant-convey screw via the opening. As a result, the slant-convey screw may not easily pick up the developer and thereby the developer may be sent back to the receive-convey screw. The sent-back developer may be accumulated on the receive-convey screw. The accumulated developer may push a developer collected from the developer carrier back onto the developer carrier, resulting in formation of a toner image having various toner densities.